This invention relates to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be delivered from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. As the molten metal formed into shells are joined and pass through the nip between the casting rolls, a thin metal strip is cast downwardly from the nip.
The casting pool is usually confined between side dams held in sliding engagement with end portions of the casting rolls so as to constrain the two ends of the casting pool against outflow. Side dams at the end portions of the casting rolls inhibit leakage of molten metal from the casting pool and maintain the casting pool at a desired depth. As the casting rolls are rotated, the side dams experience frictional wear, causing arc-shaped grooves to form in the side dams along the circumferential end portions of the casting rolls. To compensate for this wear, the side dams are movable to gradually shift inward under compression forces while having the side dams biased against the ends portions of the casting rolls in order to provide a seal with the casting rolls.
During casting operations, the metal flow rate and molten metal temperature are controlled which reduce the formation of solidified steel skulls in the casting pool in the area where the side dams, casting rolls and meniscus of the casting pool intersect, i.e. the “triple point” region. These unwanted solidified steel skulls, also known as “snake eggs” in casting, may form from time to time and drop between the side dams and the casting rolls into the cast strip passing through the casting roll nip. When these skulls drop between the roll nip, they may cause the two solidifying shells at the casting roll nip to “swallow” additional liquid metal between the shells, and may cause the strip to reheat and break disrupting the continuous production of coiled strip.
Dropped skulls, or snake eggs, may be detected as visible bright bands across the width of the cast strip, as well as spikes in the lateral force exerted on the casting rolls as they pass through the roll nip. Such resistive forces are exerted against the side dams in addition to the forces generated by the ferrostatic head in the casting pool. Skulls resulting in snake eggs in the cast strip passing through the nip between the casting rolls may also cause lateral movement of the casting rolls and the side dams. To resist the increased forces generated, bias forces have been applied to the side dams. This increases the force the side dams exert on the end portions of the casting rolls, which in turn increases side dam wear. There remains, therefore, a need to control the formation of unwanted solidified skulls in the casting pool and to reduce the formation of snake eggs in the cast thin metal strip.
Disclosed is a side dam for a continuous twin roll caster that substantially reduces the formation of solidified skulls and snake eggs. The side dam comprises a body of refractory material shaped to form a side dam and having edge portions adapted to engage end portions of casting rolls of the twin roll caster and having a nip portion adapted to be adjacent a nip between the casting rolls, with upper portions extending across the side dam to form a lateral restraint for a casting pool of molten metal during operation in a twin roll caster. The side dam also comprises a pocket between 5 and 50 mm in depth formed in the body of the side dam between the edge portions of the body, and forming shoulder portions in the body between the edge portions of the body and the pocket adapted to be worn as a casting campaign continues until the pocket is reached and continuing to be worn away at level of base portions of the pocket until casting is completed.
The shoulder portions of the body may be between 10 to 20 mm in width. In some embodiments, the shoulder portions of the body of the side dam may be between 12 to 18 mm. The pocket formed in the body may be between 5 and 35 mm in depth or between 5 and 25 mm in depth. In some embodiments, the pocket formed in the body may be between 10 and 20 mm in depth.
Also disclosed is an apparatus for continuously casting metal strip comprising: (a) a pair of counter-rotatable casting rolls laterally positioned to form a nip there between through which thin strip can be cast; (b) a pair of side dams adjacent the end portions of casting rolls adapted to confine a casting pool of molten metal supported on casting surfaces on the casting rolls above the nip, each side dam having edge portions adapted to engage end portions of the casting rolls and having a nip portion adjacent a nip between the casting rolls and upper portions extending across the side dam to form a lateral restraint for the casting pool of molten metal during operation in a twin roll caster; (c) each side dam formed with a pocket between 5 and 50 mm in depth between the edge portions and with shoulder portions between the edge portions and the pocket adapted to be worn as a casting campaign continues until the pocket is reached and continuing to be worn away at level of base portions of the pocket until casting is completed; and (d) a metal delivery system disposed above the nip and capable of discharging molten metal to form the casting pool supported on the casting rolls.
Again, the shoulder portions of the body may be between 10 to 20 mm in width. In some embodiments, the shoulder portions of the body may be between 12 to 18 mm. The pocket formed in the body may be between 5 and 35 mm in depth or between 5 and 25 mm in depth. In some embodiments, the pocket formed in the body may be between 10 and 20 mm in depth.